This invention relates to aircraft gas turbine components, and, more particularly, to the chemical composition and processing of a single crystal superalloy gas turbine shroud having improved environmental resistance and capable of use with no flow path coating.
One of the goals of aircraft engine design is to provide jet engines which operate at higher temperatures. Higher operating temperatures translate into either more efficient engine operation or more powerful engines. Operating temperatures generally are limited by the various parts making up the engine, one of which is the gas turbine shroud.
In aircraft gas turbines, air is drawn into the front end of the engine and compressed by a series of axial flow compressor stages. Fuel is injected into the air stream and the mixture is burned in a combustor. The discharge combustion gases pass through axial flow high pressure turbine stages, and low pressure turbine stages wherein energy is extracted for rotating the compressor stages. The gas then passes out of the rear of the engine.
The turbine stages are formed as stationary vanes and rotating turbine blades mounted on a turbine disk. The present invention deals with the structure of the engine in the high pressure turbine stages that are located just behind the combustors, and with the forward low pressure stages.
The high pressure turbine stages include high pressure turbine blades fixed upon cylindrical high pressure turbine disks that rotate about their cylindrical axes. The blades therefore travel in a path along the circumference of a circle. At a slightly larger radius is a stationary component known as the shroud. The shroud has several functions. First, it defines the outside of the flowpath of the hot combustion gases, acting to some extent as a seal around the hot gas flowpath. It is continuously exposed to those hot gases. Second, it aids in controlling the gas dynamics of the gas flow system and the effect of the gas stream on the rotating blades. Third, it acts as a container to minimize external damage in the event that a turbine blade fails. A shroud that performs essentially the same function may also envelop the forward stages of the low pressure turbine having a low pressure disk and low pressure turbine blades.
The shroud is an important part of the operating structure of the gas turbine engine, even though it is stationary. Such shrouds are made of high-temperature alloys, such as nickel base or cobalt base superalloys. The shroud flowpaths, which comprise the inner diameter of the shroud which may contact the turbine blades and upon which hot gases impinge, currently are coated with oxidation and corrosion resistant coatings that extend their lives in the environment of the hot combustion gases. A typical current construction for a shroud is a base structure of curved segments that slide into a circular retaining groove. Each segment is formed of a superalloy such as Mar M-509 or Hastelloy X, with an MCrAlY, an aluminide, or a ceramic coating thereupon to protect the superalloy from oxidation and hot corrosion damage. The segment may optionally have cooling apertures or holes. The term "MCrAlY" denotes an alloy of nickel, cobalt, or iron, or a mixture thereof, to which chromium, aluminum, and yttrium have been added to achieve oxidation and hot corrosion resistance. The majority of current shrouds use (Ni,Co)CrAlY coatings.
While existing shroud designs and materials operate in an acceptable manner, there is a continuing need for improved shroud construction that achieve higher temperature capability at reduced cost of construction. This improved shroud construction allows engines to operate at higher temperatures, which in turn leads to either higher performance or better fuel efficiency. The present invention fulfills this need, and further provides related advantages.